Optical fibre couplers are presently supplied in a package which is designed both to physically, protect the component and to prevent the ingress of moisture and other contaminants to the component. In one known assembly, the coupler is disposed in an inner cylindrical housing formed of two half silica tubes which are machined to tight dimensional tolerances. These half tubes are brought together and attached together with adhesive. The resultant capsule includes respective adhesive deposits which serve to anchor the optical fibre component to the inner surface of the capsule and to seal the ends through which the fibres extend. Additional adhesive may be applied to the ends of the capsule to stabilise the fibres over a wide temperature range. The capsule is placed within a stainless steel outer casing which is in turn filled with an elastomer compound. The stainless steel outer casing is provided to give additional strength and protection to the optical component assembly.
Optical fibre couplers are required to have lifetimes approaching 40 years and to withstand extensive environmental variations. Even though the silica capsule is sealed with adhesive, it has been found through intensive simulated environmental testing that these adhesives are not consistently impervious to moisture over extended periods of time. Test samples have been detected where moisture has ingressed the capsule and affected coupler performance.
It is not possible to provide a hermetic glass-metal seal with the outer metal casing because of a mismatch of thermal expansion coefficients between most metals and silica.